Method and device for thermal treatment of metal strip material



July 22, 1969 u o sAEKl ET AL 3,456,930

METHOD AND DEVICE FOR THERMAL TREATMENT OF METAL STRIP MATERIAL 3 Sheets-Sheet 1 Filed Sept. 8, 1967 FIG.

FIG. 2

D QQQ$Q July 22, 1969 KUNIO SAEKI ET 3,456,930

- METHOD AND DEVICE FOR THERMAL TREATMENT 7 OF METAL STRIP MATERIAL Filed Sept. 8. 1967 3 Sheets-Sheet 2 BAKm TEMPERATURE (C) 0 ldo 20c;

LINE SPEED /min.)

METHOD AND DEVICE FOR THERMAL TREATMENT July 22, 1969 umo sAEK| ET AL 3,456,930

OF METAL STRIP MATERIAL Filed Sept. 8, 1967 3 Sheets-Sheet 5 LACQUER FILM THCKNESS (F) m A 01 CO 26o 360 LINE SPEED /min.)

BAKING TEMPERATLRE (C) United States Patent 3,456,930 METHOD AND DEVICE FOR THERMAL TREAT- MENT 0F METAL STRIP MATERIAL Kunio Saeki, Tokyo-t0, and Arao Kamoi, Kanagawa-ken, Japan, assignors to Toyo Seikan Kabushiki Kaisha, Tokyo-to, Japan, a joint-stock company of Japan Filed Sept. 8, 1967, Ser. No. 666,366 Claims priority, application Japan, Sept. 8, 1966, 41/58,938 Int. Cl. F27b 9/28 U.S. Cl. 263-3 10 Claims ABSTRACT OF THE DISCLOSURE A metal strip passed horizontally at high line speed through an oven is supported therein in a non-contact manner by a static-pressure cushion of a treatment gas on one surface of the strip as jets of the gas are blown against another surface of the strip. By selecting the temperature of the treatment gas, either heating or cooling of a metal strip can be accomplished over a wide range of temper atures.

This invention relates generally to thermal treatment (including cooling) of metal materials in strip (hereinafter referred to as metal strip) and more particularly to a new and improved technique for effectively heating or cooling of metal strip through the use of hot or cold gas jets as the metal strip is supported in a non-contact, floating manner by a static-pressure cushion formed by some of the gas.

In heat treatment such as annealing of metal strip and baking or drying of lacquer coatings thereon, it has become a common practice to use hot gas streams as effective heating means. This trend is due to the recent wide use of readily available petroleum gases as heat sources.

Almost all of the ovens known heretofore for accomplishing rapid heating by using hot gases have had a construction wherein jets of heated gas at a temperature of from 200 to 400 degrees C. from opposed upper and lower nozzles are directed toward the strip articles to be heated at a high velocity of from 10 to 30 metres/see, (m./s.), to remove the stagnant thin layer on the surfaces of the articles and thereby to increase the coefficient of heat transfer.

However, since it has not been possible to avoid fluttering of the heated strip articles in the case where it is a metal strip material, it has been the conventional practice in almost all cases, to carry out the heat treatment operation at a line speed of from 20 to 100 metres/min, (m./min.), as a high tension of the order of from 1.0 to 3.0 kg./mm. is applied to the strip material. In addition, ovens of the so-called catenary type for effecting noncontact support of the strip material, depending on its weight, have been used.

It is an object of the present invention to provide a method and device for thermal treatment of metal strip of high treatment efilciency and high line speed whereby short treatment time can be attained.

Another object of the invention is to provide a method and device as stated above whereby metal strips of thin gauge of 0.8-mm. thickness or less can be efficiently treated without fluttering, without damage particularly to the edges of the strips, and without the necessity of applying a tension on the strip during treatment.

Still another object of the invention is to shorten the length of the oven in which the thermal treatment is carried out.

A further object of the invention is to provide economy in the consumption of heat energy (or power) in thermal treatment of metal strip.

3,456,930 Patented July 22, 19 69 According to the present invention, briefly summarized, there are provided a method and a device for thermal treatment of metal strip in which a metal strip to be treated is passed through an oven and therein supported in a non-contact manner by a static-pressure cushion of a treatment gas on one surface of the strip as jets of the gas are blown against another surface of the strip.

According to the present invention, the above thermal treatment includes either heating or cooling Within a wide range of temperatures as determined by the temperature of the treatment gas used.

The principle, and details of the invention will be more clearly apparent from the following detailed description with respect to a preferred embodiment of the invention when read in conjunction with the accompanying drawings, in which like parts are designated by like reference numerals.

In the drawings:

FIG. 1 is a schematic diagram showing the general construction of the essential components of one example of apparatus including a heat treatment device embodying the invention;

FIG. 2 is a side elevational view, in vertical cross section, showing the essential construction of the instantaneous heat-treatment oven of the apparatus illustrated in FIG. 1;

FIG. 3 is an end elevational view, in vertical cross section, of the oven shown in FIG. 2; and

FIGS. 4, 5, and 6 are graphical representations indicating various measured values obtained when baking of lacquer coatings was carried out by means of an apparatus according to the invention;

FIG. 4 indicating the relationship between baking temperature and line speed;

FIG. 5 indicating the relationship between lacquer dry film thickness and line speed and FIG. 6 indicating the relationships of gas jet velocity, dynamic pressure, and cushioning pressure to baking temperature.

Referring to FIG. 1 the principal component of the apparatus shown therein is an oven having an oven wall structure 11 and an outer heat barrier structure 12. In the space enclosed by the wall structure 11, there are provided a high-velocity gas jet head 1 and a gas cushion head 5 in mutually opposed positions with a narrow, straight-line gap therebetween. The gas jet head 1 and gas cushion head 5 have respective manifold chambers for distributively supplying hot gas to respective nozzles, the manifold chambers of heads 1 and '5 being respectively supplied with hot gas through ducts 3, 3 and ducts 7, 7.

A high-temperature blower 15 supplies hot gas from a hot-gas generating device 16 to the ducts 3, 3. Circulation ducts 8, 8, each connected at one end to the interior of the oven at one end thereof, are connected at their other end by way of circulation blowers 14, 14 to respective ducts 7, 7. Thus, the oven is capable of operating as a heat-treatment device when a metal strip 9 is passed therethrough.

As indicated in greater detail in FIGS. 2 and 3, the metal strip 9 is introduced into the oven through narrow slits in the heat barrier structure 12 and oven wall structure 11 at one end of the oven and passed through the gap between the gas jet head 1 and the gas cushion head 5, where the metal strip 9 is wafted and supported in a non-contact manner by the hot gas ejected to the bottom side of metal strip 9 through the nozzles of the gas cushion head 5 and is thereby heated. At the same time, highvelocity hot gas is blown through the velocity increasing nozzles 2 of the gas jet head 1 against the top side of the metal strip 9, which is thereby subjected to a treatment such as annealing, drying, or baking. The metal strip 9 thus treated is drawn out of the oven through narrow slits at the outlet end thereof opposite the inlet end.

As viewed in FIG. 2, in actual operation the untreated metal strip 9 is introduced continuously into the oven through the left end thereof and is moved toward the right.

' As the metal strip 9 thus enters the oven, it always carries along therewith a thin boundary layer of air at the low temperature of the outside atmosphere, which layer of air presents a resistance to the heating of the strip. To eliminate this resistance, the slot nozzles 2 of the gas jet head 1 nearest the strip inlet end are inclined in a direction whereby their gas jets are directed toward the inlet end, that is, in the direction opposite that of travel of the metal strip 9, as well as toward the metal strip 9. We have found that two slot nozzles 2 thus inclined at the inlet end of the oven are sufiicient for eliminating the above mentioned resistance to heat transfer.

One vertically directed slot nozzle is disposed adjacently downstream from the above mentioned two inlet end nozzles, and thereafter all other nozzles 2 are inclined so that their gas jets have components in the direction of travel of the metal strip 9 as well as toward the metal strip 9. By this arrangement, thrust is applied to the metal strip 9 in the direction of its travel thereby to reduce the mechanical driving power required for its handling, and, at the same time, the heat transfer toward the metal strip 9 is remarkably improved.

For preventing leakage of high-temperature gas from the oven interior through the strip inlet and outlet slits of the oven, there is provided a heat sealing system which comprises the heat barrier structure 12, the oven wall structure 11, and exhaust fans 13, 13. Since the flow velocity of the leaking gas is higher than the line speed of the metal strip 9, leakage toward the outside air would reduce the thermal efficieucy. However, by connecting the port of the circulation ducts 8, 8 near the strip inlet and outlet slits of the oven, the heat sealing effect can be increased.

The metal strip 9 is subjected to almost no tension while it is being heat treated in the oven. Therefore, the only forces acting on the metal strip 9 are the normal component of the total dynamic pressure of the gas blown against the top side of the metal strip from the gas jet nozzles and the force of gravity corresponding to the weight of the part of the metal strip 9 within the oven. As a result, since the metal strip 9 in this state is subjected to the above mentioned dynamic pressure and gravity force, it assumes a catenary shape and contacts a part of the oven interior.

Heretofore, as a measure to cope with this state in a hot-gas blast, heat-treatment apparatus, a lift force was imparted to the metal strip 9 from below by means of nozzles of similar construction to the nozzles above the top side of the metal strip 9. This lift force is imparted as a total sum of the dynamic pressures, and a hot gas flowrate, or, a dynamic pressure in excess by a value corre- 1) x 100 (percent) Vll When this relationship is considered in terms of the flowrates of the hot gas, the flowrate of the gas blown upward must be equal to /%X 100 (percent) of the flowrate of the downwardly blown gas. Furthermore, when the dynamic pressures are considered to be equal, the corresponding upward and downward velocities become equal (V [m./s.]). Accordingly, the upward flowrate must be leg-X (percent) (where k is from 10 to 20, ordinarily) of the downward flowrate.

The symbols used in the above consideration are as follows.

'Yz l vl g is the dynamic pressure (kg/m?) of the gas ejected from the lower nozzles.

In this case, the following relationship between dynamic pressures P and P is valid.

In the case where the dynamic pressures of the upper and lower nozzles are considered to be equal, the following relationship is valid.

(kg. /m.

From these relationships, appropriate selections of the process variables can be made as illustrated by the following example cases wherein, with gas temperatures of 300 and 500 degrees C., heat treatment of steel strips is carried out with a downward blowing gas blast of a velocity of 60 m./s.

When the strip thickness is 0.2 mm., the dynamic pressure P of the lower nozzles must be increased relative to the dynamic pressure P of the upper nozzles by approximately 1.5 percent at 300 degrees C. and by approximately 2.0 percent at 500 degrees C. For a strip thickness of 0.8 mm., this increase is approximately 6.0 percent at 300 degrees C. and approximately 8.0 percent at 500 degrees C.

In terms of the hot gas flowrates, the flowrate through the lower nozzles in relation to the flowrate through the upper nozzles must be, at a temperature of 300 degrees C., 100.7 percent for a strip thickness of 0.2 mm. and 103 percent for a strip thickness of 0.8 mm. At a temperature of 500 degrees C., these percentages must be 101 percent and 104 percent, respectively.

In the case where the dynamic pressures are considered to be equal, the flowrate through the lower nozzles in relation to the flowrate through the upper nozzles must be, at 300 degrees C., percent and percent for strip thicknesses of 0.2 mm. and 0.8 mm., respectively, and, at 500 degrees C., 120 percent and percent, respectively, for the two strip thicknesses.

In each of the above described general cases, dynamic pressures of hot gas are applied to the two surfaces of the metal strip 9 from the upper and lower nozzles. By the former method wherein heat treatment is accomplished by the difference in the dynamic pressures, differences in the gas velocities impinging on the metal strip 9 tend to give rise to undesirable phenomenon such as fluttering of the strip. By the latter method, the gas flowrate through the lower nozzles must be from one to two times the gas flowrate through the upper nozzles, and, therefore, there is disadvantage on the point of power economy of the hot gas supply source.

In the heat-treatment apparatus according to the present invention, a static pressure cushion is utilized as a supporting means for the metal strip to overcome the above mentioned disadvantages. This static pressure cushion is produced by a plurality of nozzles 6 disposed along the upper part of the manifold of the gas cushion head 5 as shown in FIGS. 2 and 3. More specifically, each lower nozzle 6 consists of a converging passage ending in an orifice, which may be circular or narrowly rectangular, and is inclined relative to the metal strip.

The mechanism of generation of the static pressure may be considered to be as follows. A static pressure P, (kg/m is produced in the regions surrounded by a curtain formed when hot gas ejected through the noz- Zles 6 with converging passages collides with the metal strip 9 and is reflected, and the cushioning force is imparted to the metal strip as the product of the static pressure P and the cushion area A (m?) created at the surface in contact with the curtain part and the metal strip. In FIGS. 2 and 3 the gas flow pattern in creating the static pressure is indicated by intermittent arrow lines.

Accordingly, in the case of non-contact support of the metal strip by a static pressure gas cushion, the static pressure becomes less than the dynamic pressure produced by the gas from the lower nozzles, that is, P P (kg/m and fluttering of the metal strip is automatically absorbed. At the same time, it is possible to reduce the gas dynamic pressure and flowrate.

Thus, the downward directed gas is circulated to the bottom side of the metal strip 9 by way of circulation ducts 8, 8, circulation blowers 14, 14, and ducts 7, 7, whereby the upward directed gas flow is expected for accomplishing non-contact support of the metal strip. Accordingly, another advantageous feature of the heattreatment apparatus of the present invention is that, by establishing a circulation of the hot gas from the upper nozzles 2 to the lower nozzles 6, a substantial reduction in the consumption of fuel or heat energy for actual operation at the time of operation is fully achieved.

Deflections of the metal strip 9 in the transverse direction are prevented by jet curtains formed by guide slot nozzles 10, two of which are provided on each lateral side at the upper part of the manifold of the static-pressure gas cushion head 5. This action is shown in FIG. 3 by intermittent arrow lines indicating the gas flow pattern. Accordingly, in the oven, the metal strip 9, can be handled in a smooth manner without any possibility of damage to its edges.

In order to indicate still more fully the nature and utility of the invention, the following examples of actual practice in drying and baking lacquer coated strips by means of the apparatus of the invention are set forth, it being understood that these examples are presented as illustrative only and that they are not intended to limit the scope of the invention.

Steel strips of 0.2-mm. thickness and 0.3-mm. thickness were respectively coated on both surfaces with a phenolepoxy resin lacquer applied to a thickness of 4 microns on each surface and were respectively passed through an oven according to the invention of a length of 30 metres. The 0.2-mm. strip was thus treated under the conditions of a temperature of 400 degrees C. and a line speed of 250 m./min., and the 0.3-mm. strip was thus treated at 450 degrees C. and at a line speed of 200 m./min., whereupon excellent baking results were obtained. In each case, lacquer adhesion on the strip after baking was good, and excellent formability was exhibited according to the results of a deep drawing test.

Tests relating to baking of the phenol-epoxy resin lacquer were carried out under varied conditions. Examples of the results thus obtained are as follows.

1) When, on a steel strip of 0.2-mm. thickness, the lacquer is applied at a dry film thickness of 4 microns on each surface, the relationship between baking temperature and the line speed is indicated in FIG. 4, in which reference characters A, B, and C respectively designate the regions of excessive baking, of suitable baking, and of deficient baking. In this case, lacquer coated strips were baked at line speeds of from 50 to 300 meters/min. The speed range was higher than the con ventional line speed range of from 20 to 100 meters/min.

(2) With a steel strip thickness of 0.2 mm. and a constant baking temperature of 400 degrees C. in the same oven, the quantity of lacquer applied (i.e., lacquer dry film thickness) was varied, whereupon the results indicated in FIG. 5 were obtained. In this graph, also, reference characters A, B, and C respectively designate the regions of excessive baking, of suitable baking, and of deficient baking.

In this case, also, lacquer coated strips were baked at line speeds of from 50 to 300 meters/min. This speed range was higher than the conventional line speed range of from 20 to 100 meters/ min.

(3) The relationships of various operational conditions for baking a 0.2-mm. lacquer coated strip to the baking temperature are indicated in FIG. 6. The indicated operational conditions are the hot gas jet velocity and dynamic pressure of the hot gas jet ejected from the upper nozzles and the cushion pressure of the lower nozzles necessary for enabling the coated strip to pass, as it is supported in a non-contact manner, through the oven interior.

According to these results shown in FIG. 6, it was found that the treatment could be made at a temperature of from 300 to 500 degrees C. and a gas jet velocity of from 30 to meters/second, though those of conventional conditions are respectively from 200 to 400 degrees C. and from 10 to 30 meters/second. Therefore, it is apparent that the treatment conditions of the present invention include a temperature range of from 200 to 500 degrees C. and a gas jet velocity range of from 10 to 90 meters/ second.

As illustrated by the above described examples of practice, by the practice of the present invention, it is possible to shorten the baking time of lacquer coatings and to improve the flexibility, lustre, and adhesion of the lacquer films without, moreover, causing any damage to the coated surface. Furthermore, the present invention makes possible the use of an oven line speed which is 2 to 5 times those used heretofore, whereby the operational efficiency and production efliciency can be remarkably increased.

While the invention has been described above with respect principally to heating of a metal strip, it will be apparent the device and method of the invention can be used with equal facility and effectiveness to cooling of a metal strip.

It should be understood, of course, that the foregoing disclosure relates to only preferred embodiments of the invention and that it is intended to cover all changes and modifications of the examples of the invention herein chosen for the purposes of the disclosure, which do not constitute departures from the spirit and scope of the invention.

What we claim is:

1. The method of claim 9 wherein the metal strips travel at line speeds of from 20 to 300 meters/ min.

2. In an apparatus for baking lacquer coatings on metal strip material, a thermal treatment device consisting essentially of: a thermal insulating enclosure structure provided with inlet and outlet slits respectively at opposite ends thereof for passage of a metal strip horizontally through said structure; upper gas jet nozzle means disposed above the metal strip and within the structure for directing jects of gas at a predetermined temperature against the top surface of said metal strip whereby only a dynamic gas pressure may be applied thereagainst; and lower gas cushion supply nozzle means inclined relative to said metal strip within the structure for creating a static pressure cushion of the gas at said temperature, said lower nozzles defining converging gas passages, said nozzles being positioned such that said static pressure cushion imparts a lifting force to said strip to balance the downward force of the dynamic gas pressure imparted by said upper gas jets and the weight of said strip and to support said strip in a non-contact manner as fluttering of the strip due to said upper gas jets is automatically suppressed, thermal treatment of the strip thereby being accomplished without any contact between the strip and a solid part of the device.

3. The device of claim 2 wherein said upper gas jets are directed against the upper surface of said metal strip at an angle substantially equal to 90 relative to said surface.

4. The thermal treatment device as claimed in claim 2 in which the upper gas jet nozzle means comprises an upper gas manifold supplied with the gas under pressure and a plurality of jet nozzles disposed in a row extending between points respectively in the vicinity of said inlet and outlet slits, each jet nozzle having an inlet for receiving the gas from the upper gas manifold and an outlet confronting the upper surface of the metal strip in close proximity thereto, said outlet being in the form of a narrow slot extending horizontally and transversely with respect to the metal strip, and the lower gas supply means comprises a lower gas manifold supplied with the gas under pressure and a plurality of gas cushion nozzles disposed in a row in substantially opposed relationship to the row of jet nozzles of the upper gas jet means, each gas cushion nozzle having a curved converging passage supplied with gas from the lower gas manifold and terminating at a narrow substantially annular outlet for directing the gas in a direction to create the static-pressure cushion in cooperation with the other gas cushion nozzles.

5. The device of claim 4 wherein at least one of the upper gas jet nozzles nearest said inlet end is inclined in toward said inlet end whereby gas jetted therefrom is directed in a direction opposite that of travel of the strip and the remainder of the upper gas jet nozzles are inclined toward said outlet end whereby gas jetted therefrom is directed in the same direction as that of travel of said strip.

6. The device of claim 4 wherein guide gas jet nozzles 8 are positioned at each lateral side of said lower gas manifold supplied with said gas positioned so as to form gas jet curtains at each side of said strip but not impinging on the lower surface thereof whereby deflections of said strip in a transverse direction are prevented.

7. The device of claim 4 wherein said narrow substantially annular outlets of said gas cushion nozzles are substantially circular in shape.

8. The device of claim 4 wherein said narrow substantially annular outlets of said gas cushion nozzles are substantially rectangular in shape.

9. A method for baking lacquer coatings on metal strip material consisting essentially of the steps of causing a metal strip to travel horizontally in the longitudinal direction thereof, directing jets of gas at a predetermined temperature against the upper surface of said travelling metal strip such that the only force exerted on said upper surface by said gas consists of the normal component of the dynamic pressure thereof and simultaneously directing jets of gas at said temperature to form a static-pressure cushion of gas at the lower surface of said travelling metal strip such that the only force exerted on said lower surface by said gas consists of the normal component of the static pressure thereof, said static-gas cushion imparting a lifting force to said strip which balances the downward force imparted thereto by said dynamic pressure force and the weight of said strip, whereby said strip is supported in a non-contacting manner and fluttering of said strip due to said dynamic pressure force is automatically suppressed, baking of the strip being accomplished without contact between the strip and a solid object during the baking.

10. The method for thermal treatment of metal strip material as claimed in claim 9 in which the gas is at a temperature of from 200 to 500 degrees C., and the jets thereof are directed against the top side of the metal strip at an angle substantially equal to degrees relative to said surface and at a velocity of from 10 to 90 meters/ second.

References Cited UNITED STATES PATENTS 2,409,431 10/1946 Hess 2633 2,862,305 12/1958 Dungler 34-455 3,048,383 8/1962 Champlin 2633 3,181,250 5/1965 Vits 34156 X JOHN J. CAMBY, Primary Examiner 

